During operation of an internal combustion engine, it is desirable to control the formation and emission of certain gases, such as the oxides of nitrogen (NO.sub.x). One method of achieving this result is the use of EGR which is a process whereby exhaust gases are selectively routed from the exhaust manifold or manifolds to the intake manifold of the internal combustion engine. The use of EGR reduces the amount of NO.sub.x produced during operation of the internal combustion engine. In particular, NO.sub.x is produced when nitrogen and oxygen are combined at high temperatures associated with combustion. The presence of chemically inert gases, such as those gases found in the exhaust of the engine, inhibits nitrogen atoms from bonding with oxygen atoms thereby reducing NO.sub.x production.
However, EGR is only needed during certain engine operating conditions. Hence, a valve, commonly referred to as an EGR valve, is used to selectively route a portion of the exhaust gases from the exhaust manifold or manifolds to the intake manifold. With the use of microprocessors, an engine control module can rapidly process a number of sensor inputs to determine when the use of EGR would be most advantageous. Moreover, operating conditions often change quickly during normal operation of the engine thereby requiring the EGR valve to open and close rapidly.
The EGR valve must also be "fail safe". More specifically, the EGR valve should remain in a closed position if any of the components associated therewith (e.g. the engine control module) fail or otherwise become inoperable. Such fail safe operation generally requires use of a spring that biases the valve into the closed position. The magnitude of the spring bias must be large enough to hold the valve closed during extreme engine operating conditions such as when the exhaust gases within the engine exhaust manifold is at or near its maximum pressure. An actuator, such as a solenoid, selectively provides a force to overcome the spring bias in order to open the EGR valve. It should be appreciated that the time period necessary for the solenoid to overcome the spring bias of the spring is proportional to the magnitude of the spring bias. In particular, the time period necessary for the solenoid to overcome the spring bias of the spring increases as the magnitude of the spring bias increases. Hence, a tension exists between the desirability of rapid opening and closing of the EGR valve and the desirability of fail safe operation. Such tension is a drawback associated with EGR valves which have heretofore been designed.
Also, many engines used in heavy machinery, such as earth moving equipment, are turbocharged diesel engines. In a turbocharged diesel engine, it is often advantageous to divide the exhaust manifold into two smaller manifolds. Each of the two smaller manifolds routes a portion of the exhaust gases therein to a separate inlet on opposite sides of the turbocharger turbine disk. Applying exhaust gases to the turbine disk in such a manner accelerates the turbine disk more rapidly relative to applying all of the exhaust gases to only one side of the turbine disk. Such rapid turbine acceleration allows the engine to respond more quickly to increased load conditions. A drawback to the use of EGR with two separate exhaust manifolds is that exhaust gases are desirably extracted from each exhaust manifold equally. In particular, an equal amount of exhaust gases are desirably extracted from each of the exhaust manifolds in order to place an equal load on each half of the engine. Thus, it is desirable for the EGR valve or valves to extract exhaust gases equally from each of the exhaust manifolds.
What is needed therefore is an apparatus and method for selectively routing EGR gases which overcome one or more of the above-mentioned drawbacks.